E.F. and M.F. Lab – Magnesium Chloride

Procedure:

1)       Obtain a clean, dry conical flask and weigh it to the nearest 0.01 g. Record this value on the report form.

2)       Place a small piece of magnesium ribbon into the flask and record the mass on the report form. From the difference in masses, record the mass of magnesium used. Note: The mass of magnesium should not exceed 0.15 grams, or the product may be difficult to dry

3)       Measure 7 mL of 2 M HCl* in a 10 mL graduated cylinder (or you may use a pipet) and carefully add the HCl solution to the flask containing the Mg ribbon (Caution: Vigorous reaction!). Allow the reaction to proceed until the reaction is complete, giving a clear solution with no magnesium particles remaining.

4)       Place the flask on an electric hot plate and heat to nearly boiling. Avoid excessive heat that can cause dangerous splattering of hot HCl!

5)       Heat the solution until evaporation of the water is complete. The white, solid product that remains is magnesium chloride. It is difficult to tell by appearance when the product is completely dry, so we will use a method called heating to constant weight. When the product appears thoroughly dry, carefully remove the hot evaporating dish from the hot plate (you may use crucible tongs to handle the hot dish), allow it to cool, and record the weight on the report sheet under “first weighing.” Then, place the dish back on the hot plate and heat for 10 additional minutes. Allow the dish to cool and record the weight under “second weighing.” If the second weighing agrees with the first weighing, you may reasonably assume that drying is complete. If the second weighing is less than the first weighing, place the dish back on the hot plate and heat for 10 more minutes and obtain a third weighing. Repeat this process until successive weighings agree to within 0.01 g.

6)       From your data, calculate the moles of Mg and the moles of Cl in the product. From the number of moles of each element, determine the empirical formula of magnesium chloride.

7)       Wash your evaporating dish with water (the product may be washed down the sink) and return your equipment to their proper storage locations before leaving the lab.

* The “M” in “2 M HCl” stands for molarity. This is a standard concentration unit in chemistry, and it means moles of solute per liter of solution, mol/L. One liter of 2 M HCl solution contains 2 moles of HCl.